Energy-state-selected cesium beam intensifier

ABSTRACT

A cesium-beam atomic frequency standard in which the beam from the effuser is reflected from a reflector target surface placed between the concave and convex polepieces of the A-magnet energystate selector. The surface is coated with a colloidal suspension of graphite. The reflection is of the specular-type and the glancing angle may be between 2* and 10*. Condensation of atoms on the reflector surface is avoided by heating the reflector to approximately the temperature of the cesium source.

United States Patent [1 1 George Aug. 26, 1975 [54] ENERGY-STATE-SELECTED CESIUM 3,328,633 6/1967 George 331/94 x BEAM INTENSIFIER [75] Inventor: James George, Swampscott, Mass. P m ry m r -Siegfrid H. Grimm A ,A F R.S. ";P.S [73] Assignee: The United States of America as Home), m or [rm scmscm chnelder represented by the Secretary of the Navy, Washington, DC. [57] ABSTRACT [22] Filed: Aug. 21, 1974 Appl. No.: 499,255

US. Cl 331/94; 331/3 [51] Int. Cl. H01S l/00 [58] Field of Search..... 331/94, 3; 324/.5 R, .5 MA, 324/.5 H

[56] References Cited UNITED STATES PATENTS 2,972,115 2/1961 Zacharias et al. 331/3 tMjCs ATOMS A cesium-beam atomic frequency standard in which the beam from the effuser is reflected from a reflector target surface placed between the concave and convex polepieces of the A-magnet energy-state selector. The surface is coated with a colloidal suspension of graphite. The reflection is of the specular-type and the glancing angle may be between 2 and 10. Condensation of atoms on the reflector surface is avoided by heating the reflector to approximately the temperature of the cesium source.

4-Cl aims, 1 Drawing Figure A -MAGNET ENERGY-STATE SELECTOR CONVEX POLEPIECE E CONCAVE PbLEPlECE E ENERGY-STATE-SELECTED CESIUM BEAM INTENSIFIER BACKGROUND OF THE INVENTION .bcam resonator.

In general, the following qualities determine the frequency stability capability factor, S, for a Cs-beam resonator: I. the Cs 4,0 3,0 resonance linewidth at halfbeam intensity; 2. the Cs 4,0 3,0 resonance frequency; 3. the total output signal; 4. the Cs AC resonance error signalv Presently, Cs-beam frequency standards yield short-term stabilities of the order of 6 parts in 10 for one-second measurement intervals.

For practical reasons, the achievements of optimum characteristics for the factors mentioned above are restricted by physical size, weight and reliability. Present Cs resonators are less than 50 cm. long and yield Cs linewidths between 500 and 600 Hertz. Since the Cs resonance linewidth is determined by the atom/microwave interaction time, the reduction or narrowing of the 4,0 3,0 linewidth can be achieved either by utilizing lower velocity Cs atoms, by using longer microwave structures, or both. The use of longer microwave structures cm) may present some undesir able longterm effects, i.e., distortions from C-field inhomogeneity, greater Csbeam divergence, and increased Cs-beam scattering in addition to contending with larger physical sizes.

The utilization of the lower side of the MaxwellBoltzman velocity distribution is not practical with present Cs resonator techniques. The quality of the signal is very poor because the Cs-atom population as a function of atom velocity is small. In addition, the quality of the Cs metal itself plays an important role in the function of atom intensity as a function of atom velocity. Since Cs is chemically active, significant quantities of absorbed gases can exist in solution with Cs and are liberated upon applying heat to the Cs source. The Cs atoms in the gaseous phase experience many Cs atomresidual gas atom collisions behind the effuserv The lower velocity atoms are so scattered that the geometrical optics of the source do now allow their passage through the effuser. Although the atom intensity at these lower velocities can be increased by increasing the source temperature, the oven-scattering effect increases along with an increase of expended effused Cs, leading to resonator operating problems and reliability. Further, the Cs resonance linewidth is increased slightly.

An increase in Cs atom signal can also be achieved by selecting the more populated portion of a normal distribution. This is accomplished by increasing the efficiency of energy-state selection for a given Cs resonator. However, selecting the most probable beam velocity is not particularly advantageous because of the associated increase in Cs resonance linewidth.

SUMMARY OF THE INVENTION The present invention accomplishes its desired results by utilizing the principle of specular reflection of a cesium beam to increase the number of desired atoms in the reflected, energystate-selected beam. A Cs atom reflector is placed between the A-magnet of a Cs resonator to reflect the beam of atoms coming from the effuser.

OBJECTS OF THE INVENTION An object of the present invention is to increase the number of desired, energy-state-selected atoms coming through the A-magnet section of a Cs-beam atomic resonator.

A further object is to increase the number of desired, energy-state-selected atoms arriving at the collector means of a Cs-beam atomic frequency standard.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE shows a partial view of a cesiumbeam frequency standard including an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The figure shows the fore end of a cesium-beam resonator as modified by the present invention. The figure shows the Cs-beam effuser 12, in the Cs source 10, the concave polepiece l4 and the convex polepiece 16 of the A-magnet. The rest of the device is conventional and consists of rf field-generating means, the B- magnets and atom collecting means. The A- and B- magnets constitute energy-state selection means. (The convexity and concavity of the polepieces is apparent only to an observer looking at them along the optic axis of the resonator. The optic axis extends from left to right in the figure and is parallel to the horizontal, parallel edges of the polepieces.)

What is different from the usual Cs-beam frequency resonator is the fact that the Cs beam is reflected from a heated reflector 18 to slow it down, rather than being projected through the magnets along the optical axis. Reflection is accomplished by the upper surface of the reflector which may be made of metal such as steel or aluminum, the reflecting surface 20 being coated with a colloidal suspension of graphite.

The beam from the effuser, which is directed toward the concave polepiece where the reflector is located, has plus and minus Cs atoms (the plus and minus referring to the different directions of spin of the atom). The A state-selector magnet eliminates the positive atoms and only the minus atoms are reflected.

It has been found that heating of the surface of the reflector to approximately the temperature of the Cs source helps to prevent condensation of the impinging atoms, thereby increasing the reflection of the atoms which increases the intensity of the desired atoms, i.e., the desired atom population is greatly increased at the collector means. Condensation of atoms on the reflector surface leads to diffuse scattering of the atoms and therefore a lower population of atoms at the collector means. The temperature of the Cs source, and therefore, that of the reflector target 18, is approximately 373K; the reflector temperature is not critical. A heater element 22 is used to heat the target and the temperature thereof may be sensed by a thermistor temperature probe 24, for example Specular-type reflection has been observed with glancing angles of from 2 to 10.

An increase in Cs-beam atoms at the collector of 13 times has been observed with the reflector and heater as compared with conventional Cs-beam resonators, although present theory holds that only diffuse reflections can occur. However, the detection of an enhanced signal, the sharp directivity of the reflected beam, and the achievement of very good flop signal-tonoise ratios :1) indicate an effect similar to specular-type reflection.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a cesium-beam resonator including a cesium atom source, a cesium beam effuser and a-magnet energy-state-selector means having a concave polepiece and a convey polepiece, means for reflecting the cesiumatom beam from the effuser,

said reflecting means being located adjacent said concave polepiece and having a surface from which said beam is reflected.

2. In the cesium-beam resonator of claim 1, heating means for heating said reflecting means.

3. In the cesium-beam resonator ofclaim l, a coating applied to the reflecting surface of said reflecting means, said coating comprising a colloidal suspension of graphite.

4. In the cesium-beam resonator of claim 2, said heating means being adjusted to heat said reflecting means roughly to the temperature of said cesium-atom source. 

1. IN A CESIUM-BEAM RESONATOR INCLUDING A CESIUM ATOM SOURCE, A CESIUM BEAM EFFUSER AND A-MAGNET ENERGY-STATESELECTOR MEANS HAVING A CONCAVE POLEPIECE AND A CONVEY POLEPIECE, MEANS FOR REFLECTING THE CESIUM-ATOM BEAM FROM THE EFFUSER,
 2. In the cesium-beam resonator of claim 1, heating means for heating said reflecting means.
 3. In the cesium-beam resonator of claim 1, a coating applied to the reflecting surface of said reflecting means, said coating comprising a colloidal suspension of graphite.
 4. In the cesium-beam resonator of claim 2, said heating means being adjusted to heat said reflecting means roughly to the temperature of said cesium-atom source. 